I have a colleague who believes that the number in brackets after DA on a Jeppesen approach plate is the ACTUAL rad alt reading at DA/MDA. I say it is the height above the runway reference (i.e. QFE equivalent).
I say you cannot use Rad Height as a guide to DA on a Cat 1 ILS. He says you can.
Who is right?
You are right. The number in parentheses on a Jepp chart is HAT (Height Above Touchdown Zone Elevation) See elsewhere in this thread for Touchdown Zone Elevation (TDZE). Radar altimeter is not a permitted means (and not an appropriate means) of determining DA on a CAT I ILS. (A CAT I ILS could potentially be designed to use radar altitude, but they are not, in part, because this would require a radar altimeter.) And on a non precision approach you may be bumping along at the MDA for some distance before reaching the MAP; the radar alt may be all over the place. On Cat II & III approaches radar altimeter is used and "RA" is shown in the minimums section of the chart. This is according to U.S. TERPS. Lots going on here; a link if you want to dig in:
http://www.faa.gov/documentLibrary/m...s_1-26_rev.pdf Some differences may apply in other jurisdictions.
I found it useful to study TERPS carefully early in my instrument flying. Recommend that all instrument pilots at least give it (or the equivalent where you fly) some study, however, far more detail than you will need to keep in mind. What I found most useful is an understanding of the obstacle clearance provided (usually not much!) in critical phases of flight.